1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier and a laser printer.
2. Description of the Related Art
An image forming apparatus (for example, copier and printer) using an intermediate transfer member forms toner images on a photosensitive drum through a developing roller, and transfers the toner images onto the intermediate transfer member (hereinafter referred to as “primary transfer”). After that, the toner images are collectively transferred onto a recording material such as paper (hereinafter referred to as “secondary transfer”), to thereby obtain an image. In the image forming apparatus, in a case where the recording material is fed into the image forming apparatus and conveyed to a secondary transfer position, a slip occurs depending on an amount of stacked sheets inside a cassette, a doubly-fed amount due to a preceding sheet, an abrasion state of a sheet feeding roller, a type of media, or the like, to thereby cause a variation. The variation means that a sheet arrives at a predetermined position at a varied timing. When the variation occurs during conveyance of the recording material from a start of sheet feeding up to the arrival at the secondary transfer position, the toner image cannot be transferred onto an appropriate position of the recording material, and a high-quality image cannot be formed.
In a conventional technology, for example, the following control is performed. That is, a sheet detection sensor provided on a conveying path up to the secondary transfer position is used to measure a varied time period taken by the recording material being conveyed, and conveyance control is carried out based on the measured time period. Thereby, the control is performed so that the toner image is transferred onto the appropriate position of the recording material (see, for example, Japanese Patent Application Laid-Open No. 2007-101666). Specifically, the varied time period is compared with a reference time period without the variation, and a conveyance speed is increased or decreased based on a difference therebetween to convey the sheet up to the secondary transfer position. Under such control, it is possible to cancel the variation to obtain a satisfactory image. Such control is hereinafter referred to as “conveyance speed control”.
In the conventional technology, a timing to carry out the conveyance speed control is determined only based on a timing at which the recording material is detected by the sheet detection sensor. In that case, for example, when recording materials are continuously conveyed in the image forming apparatus for conveying the recording material from the start of the sheet feeding up to the arrival at the secondary transfer position by using one driving source, the following case may occur. That is, when the conveyance speed control is carried out at the timing at which the record material is detected by the sheet detection sensor as in the conventional technology, there may be a change in the conveyance speed of the preceding sheet being subjected to the secondary transfer, which may hinder normal image formation. Therefore, in order to avoid affecting the conveyance of a preceding recording material, the conveyance speed is controlled by increasing a sheet feeding interval so as to constantly guarantee that a subsequent sheet arrives at the sheet detection sensor after the preceding sheet has passed through a conveyance portion located at the most downstream position among those operated by the same driving source. However, this control increases an interval of image formation, which lowers productivity of the image forming apparatus.